<!DOCTYPE html>
<html>
<head lang="en">
    <meta charset="UTF-8">
    <title></title>

</head>
<body>
    <select name="select" id="ssn" onchange="fun()">
        <option value="s">选择你的出生日期</option>
        <option value="w">1990</option>
        <option value="f">1991</option>
        <option value="h">1992</option>
        <option value="n">1993</option>
        <option value="y">1994</option>
        <option value="r">1995</option>
    </select>
<script>
    alert(document.getElementById("ssn").length);
    function fun(){
        alert(document.getElementById("ssn").value);
    }
</script>
</body>
</html>